The old, obsolete Firefox specific drag and drop API events dragdrop and draggesture are no longer supported. Be sure to update any code still using them to use the HTML drag and drop API (error 1162050.

Pointer Lock API

Before Firefox 50, requestPointerLock() asked for permission using a doorhanger, and pointer lock would not be enabled until the user granted permission. From Firefox 50, pointer lock is like the fullscreen API: it's granted immediately, but a notification is displayed explaining to the user how to exit (error 1273351).

The MediaStreamTrack methods getConstraints() and getSettings() have been implemented; these let you get the most recently applied set of customized property constraints and the actual values of all of the track's constrainable properties, respectively. The accompanying data types have been documented as well.

The RTCDataChannel.stream property has been removed. This was replaced with RTCDataChannel.id in Firefox 24, but was supported for backward compatibility. Please be sure to update your code to use the id property if you haven't done so yet.

Audio/Video

The AlignSetting enum (representing possible values for VTTCue.align) incorrectly previously included the value "middle" instead of "center". This has been corrected (error 1276130).

The non-standard and experimental method HTMLMediaElement.seekToNextFrame() now seeks to the next frame in the media asynchronously, rather than synchronously, and returns a Promise which resolves once the seek is complete.